1. Field of the Invention
The present invention relates generally to the field of spectrophotometry and more particularly to the compensation of sample measurements for stray light.
2. Description of Related Art
Spectrophotometers can be described as instruments that measure the relative amount of radiant energy absorbed or transmitted by a sample for one or more radiation wavelengths. Such instruments generally include a continuum radiation source, i.e., one which generates radiant energy over a relatively broad band of wavelengths. For a so called "scanning" spectrophotometer, a schematic diagram of typical of the prior art is shown in FIG. 1, monochromator 10 receives the radiation 13 from the source 12 and isolates an output beam 14 comprising radiation having wavelengths substantially within a relatively narrow wavelength band. In the particular example shown, the monochromator 10 includes an entrance slit 16 which receives the radiation of the source 12 through a focusing lens 17, a collimating lens relaying parallel radiation onto a rotatable disperser 20 which breaks up the radiation into its wavelength components spatially, and a second collimating lens 22 which focuses the radiation onto an exit slit 24. Depending on the tilt of the disperser 20 and/or the opening in the exit slit 24, a particular band of radiation can be selectively output by the monochromator 10.
The monochromator output beam 14 is directed to a detector 26 which produces an electrical output having a value related primarily to the radiant power received by the detector and the spectral sensitivity of the detector. In the absence of any radiation falling on the detector, a relatively small signal called the "dark current" may arise from either the detector or the associated electronics. This "dark current" is easily determined by blocking the beam and measuring the output signal. The difference between the total signal and the dark current is called "detected radiant power" (DRP) and is the portion of the detector output generated only in response to incident radiant power.
The beam path 14 between the monochromator 10 and the detector 26 is accessible to the user of the instrument so that sample or reference materials can be place into the beam in a cell 28. Usually, the relative transmittance or absorbance of a sample with respect to a reference material is measured. For example, for given radiation wavelengths within the narrow wavelength band, the reference material is placed into the beam 14 between the monochromator 10 and the detector 26 and the resulting DRP is measured. The reference material is removed and, with the sample in its place, a second DRP is measured. The sample transmittance is then expressed as a ratio of the second (sample) DRP with respect to the first (reference) DRP. Absorbance is related to transmittance by the conversion expression A =-log T, where A is absorbance and T is transmittance. It will be recognized that although various examples and discussions included herein are presented in terms of transmittance, such examples and discussions are equally applicable to the measurement of absorbance since absorbance and transmittance are related terms for the same phenomenon.
Referring to FIG. 2, the monochromator wavelength band may be largely defined by two parameters, half band width 32 and central wavelength 34. The half bandwidth 32 is generally defined as the wavelength interval at which the DRP of the narrow wavelength band 31 is one-half the maximum DRP in the band. The half bandwidth 32 is usually dependent upon the width of monochromator entrance and exit slits 16 and 24 through which the continuum radiation 13 and the output beam 14 pass, respectively. The central wavelength 34 is the wavelength corresponding to the maximum DRP. Ideally the DRP should fall to zero at wavelengths equal to the central wavelength 34 plus and minus a half band width value, i.e. at the points indicated by 37. In practice this is never the case. Two factors complicate this ideal situation. The shape of the DRP versus wavelength graph, (FIG. 2), called the "slit function", may not be triangular and a small DRP may arise from radiation of wavelengths at 39 far removed from the central wavelength 34 as will be explained below.
In some types of spectrophotometers the central wavelength 34 is determined by means of a mechanism that rotates the disperser 20 within the monochromator. This mechanism may incorporate a dial or digital readout intended to display the central wavelength corresponding to the maximum DRP. It is important to recognize that this dial reading does not necessarily correspond to the actual central wavelength of maximum DRP. The difference between the dial reading of wavelength and the actual central wavelength of maximum DRP is usually called the wavelength error. While effort is made to minimize this wavelength error, it may become significant, particularly near the extremes of disperser rotation. In this discussion a distinction will be made between the dial reading of wavelength and the actual central wavelength.
Ideally, the monochromator 10 should pass only radiation having wavelength within the narrow wavelength band, that is, the monochromator output beam 14 should be free of radiation with wavelength outside of an interval 37 twice the width of the half bandwidth 34 and centered at the dial setting wavelength 36. However, such ideal monochromators do not exist. In addition to radiation with wavelength within such an interval 37, which has been called "primary radiation", the monochromator output also includes radiation at wavelengths 39 outside the interval of primary radiation. Such radiation has been referred to in the art as "stray light" and is often a result of light scattering by the disperser 20 within the monochromator 10. The ratio of detected stray radiation to the total detected radiation is known as stray radiant power ratio (SRPR).
The spectral and spatial character of the primary radiation follows the laws of diffraction, i.e. specific portions of the beam are directed to the exit slit or to an equivalent array detector element. On the other hand, the stray radiation is broadly scattered everywhere within the monochromator. The direction of scatter is little affected by the spectral character of the radiation passing the entrance slit. It is not intended to say that the scattered rays uniformly illuminate the inside of the monochromator. Rather the area near the inside of the exit slit (or the detector array in a diode array instrument) is illuminated by stray (white) light much like the space in front of an automobile headlight. Intensity of radiation decreases as the angle between the scattered and diffracted rays increases.
While "stray light" is used herein as just described for the purpose of discussion of the prior art, it will be recognized by those skilled in the art that the term "stray light" has not been clearly defined or limited in use in the prior art. The term has been used variously to denote the overall problem of stray light in spectrophotometers, or a measured quantity of stray light, usually unknown units, or dimensionless ratio such as SRPR. It will also be noted that "light" is synonymous with "radiation" and that "radiation" as used herein includes electromagnetic radiation throughout the ultraviolet, visible and infrared wavelength regions.
Stray light is usually of interest in two contexts. First, it is a general practice of spectrophotometer instrument manufacturers to measure stray light for a particular spectrophotometer and to publish the measurement as an instrument performance specification. Periodically, a spectrophotometer should be retested to determine whether the spectrophotometer till meets the specification. A failure of the instrument to do so is an indication that the spectrophotometer performance may have degraded and that service may be required.
A second context is the measurement of sample transmittance where it is desirable to compensate for the effects of stray light. In the past, stray light induced error has been reduced by limiting the wavelength interval of detectable radiation by means of blocking filters and choice of sources. The blocking filter 30 is positioned between the sample cell 28 and the detector 26. The filter should be highly transmitting to the desired radiation wavelength yet absorb much of the stray radiation. Typically, the bandwidth of a blocking filter is much wider than the half bandwidth or resolution of the monochromator so that one blocking filter may cover an interval of many half bandwidths. Referring to FIG. 2 the dotted lines 38 in the figure are representative of the distribution of detected radiation in the presence of a blocking filter. This method reduces both scattered and multiple order diffracted radiation. However, the blocking filter alone does not, usually, reduce the stray light error as much as is desired and both blocking filter and compensation methods are employed simultaneously.
U S. Pat. No. 4,526,470 issued to the same inventor and assigned to the same assignee as the present invention and the article "Stray Radiation" by the inventor published in Advances in Standards and Methodology in Spectrophotometry, Elsevier Science publishers (1987), pages 257-275, describe different approaches of measuring and compensating for stray light. While these methods have been successful in compensating stray light in conventional scanning spectrophotometers, i.e. one which utilizes a monochromator to isolate an output beam of radiation within a relatively narrow wavelength band, compensation of stray light is inherently more difficult in diode-array instrumentation.
In a diode-array spectrophotometer, the broad band of radiant energy of the radiation source is directed through the sample, with the transmitted radiation dispersed into a spectrum by a grating before being directed to an array of photodiodes, i.e. the transmitted radiation is broken up spatially into its wavelength components which are diffracted by an amount according to the wavelength values. Each diode in the array is exposed to a small wavelength interval of the entire spectrum. The perceived wavelength is dependent on the location of the respective diode. Each diode element detects the transmitted radiant intensity of a small wavelength interval to provide an indication of the absorbance of the sample component at the respective wavelength as identified based on the location of the diode element. The overall bandwidth and the spatial resolution of the diode array will depend on the number and size of each discrete element, their spacing, and other optical parameters of the instrument. In order to obtain a broad bandwidth as well as a high resolution, a large number of small diode elements at close spacing are required.
It is difficult to compensate for stray light in a diode array spectrophotometer. As will be described more fully hereinbelow in the detailed description of the present invention, the most important and desirable feature of the diode array instrument is the array. It allows the simultaneous measurement of a broad spectrum of the light source and it has no moving element. However, the desire to process the detected radiation at all wavelength simultaneously frustrates the use of blocking filters to reduce stray light. Since the sample is placed further upstream in the optical path (upstream of the disperser) as compared to conventional scanning spectrophotometers (sample placed downstream of the disperser), this makes the measurement of sample transmittance with respect to wavelengths sensitive to the refractive index and optical path of the sample. It is difficult to insert a blocking filter into the optical configuration without risking abridging the spectral interval. Blocking filters have to be located immediately in front of the detector elements and must be very thin to avoid defocusing or shadowing of adjacent elements. The extremely small size and separation of detector elements exacerbates this problem.
When one uses a diode array, ideally one would like detectable radiation falling on each element in the absence of the sample to be constant and independent of wavelength throughout the range of the instrument. In some instruments, this is partially accomplished by tailoring the length of the diode elements. While this facilitates signal processing it does not reduce the stray light component.
Furthermore, stray light from second and higher order diffractions at the grating can be particularly troublesome. This can be so bad as to force use of one or more blocking filters in spite of the difficulty. One prior method involves limiting spectral intervals to less than one grating order and changing the blocking filter for each order. This partially defeats the goal of a fast scan and still leaves a high stray light unless compensation for stray light is also employed.